Dipsas andiana (Boulenger, 1896)

CADLE, JOHN E. & MYERS, CHARLES W., 2003, Systematics of Snakes Referred to Dipsas variegata in Panama and Western South America, with Revalidation of Two Species and Notes on Defensive Behaviors in the Dipsadini (Colubridae), American Museum Novitates 3409, pp. 1-48 : 32-42

COMPARISON OF DIPSAS ANDIANA AND DIPSAS NICHOLSI

As currently understood, Dipsas andiana and D. nicholsi are definitely known only from western Ecuador and central Panama, respectively (map 3). Our recognition of D. andiana as the proper name of Ecuadorian specimens previously referred to D. variegata nicholsi requires us to compare these two species in greater detail. In fact, Dunn had recognized the similarity between D. nicholsi and D. andiana when he described the former: ‘‘The nearest approach in color [to D. nicholsi ] seems to be andiana , which has one lower labial pair in contact behind men­ modern methods of spraying, packing, and shipping bananas has drastically curtailed such traffic. But at least five specimens of Dipsas andiana found their way to New York in the period 1939–1958. AMNH catalog localities (‘‘Guatemala’’, ‘‘NW Colombia’’) for a few specimens were conjectures, and we suspect that all derived from banana shipments out of Ecuador.

A banana stowaway (AMNH 79013) received from the Bronx Zoo contains two intact pulmonate gastropods (minus shells) that had been fed to the snake in captivity. Oliver (1955: 70–71, under the name D. v. nicholsi ) noted that banana stowaways acquired by the Bronx Zoo learned to prefer snails that had been removed from their shells, showing ‘‘a marked disdain for those still in the shell’’. Neither of the two Bronx Zoo Dipsas andiana shown in a photograph made their way into the AMNH collection, but one of them resembles AMNH 79013 in having an ‘‘atypical’’ head pattern in which the cephalic marking is confluent with the neck blotch on

Map 3. The widely separated distributions of Dipsas nicholsi in central Panama and Dipsas andiana in western Ecuador. Based on locality records in maps 1 and 2.

tal and whose locality is quite remote’’ ( Dunn, 1933: 194).

DIAGNOSTIC DIFFERENCES: Distinctive characters shared by the two species include (figs. 5, 13): (1) unusual Π­ shaped marking on the head; (2) similar dorsal patterns, including uniform light brown ground color and dark round or elliptical blotches with narrow pale borders; (3) relatively unmarked venters (variable in D. andiana ); (4) relatively high numbers of ventrals and subcaudals (tables 1, 2); and (5) similar overall body form and size, although tables 1 and 2 indicate that D. nicholsi may attain a somewhat larger size than D. andiana . We now know that the differentially diagnostic character of the infralabial scales mentioned by Dunn (1933) is variable within both species (see descriptions), as recognized later by Dunn for D. nicholsi (1940: 117) . Nonetheless, several scutellation, coloration, and hemipenial characters distinguish D. nicholsi and D. andiana .

Male Dipsas nicholsi have significantly higher ventral counts (tables 1, 2) than male D. andiana (t = 4.09, df = 9, p K 0.01; male subcaudal counts not significantly different). The ventral and subcaudal counts of the single female of D. nicholsi are significantly different (p K 0.01 in each comparison) from the female sample of D. andiana (ventrals: t = 7.38, df = 4; subcaudals: t = 15.4, df = 3). In addition, several subtle aspects of color pattern distinguish the two species (characters following are given for D. nicholsi first, D. andiana second; see figs. 4–5 vs. 13–14): (1) All dorsal blotches wider than tall vs. blotches taller than wide. (2) Blotches broad­ er vs. narrower. This is most easily seen at midbody and just anterior to the vent, where blotches are 4.5–6 and 5–7 scales wide, respectively, in D. nicholsi vs. 3–4 and 2.5– 3.5 in D. andiana . This difference corresponds to a tendency toward greater relative width of the interspaces (compared to the blotches) in D. andiana , whereas in D. nicholsi the interspaces and blotches are more nearly the same. The difference is most easily seen by comparing patterns on the posterior body in the two species. (3) Branches of the Π­ shaped marking on the head continue onto the neck as broad, elongate streaks fused to the first pair of neck blotches vs. marking ends on the head and usually not connected to neck blotches (cf. figs. 4, 8; but see fig. 15 for one exception in D. andiana ). (4) Branches of the head marking continually diverging vs. more or less parallel (compare figs. 4, 8, especially the orientation of the inner margins of the markings on the posterior part of the head). The difference in configuration results in a different scale count between the branches of the marking at the level of the mouth commissure: the branches in D. nicholsi are separated by 4–5 scales at this level, whereas those in D. andiana are separated by only 2–4 scales.

A few more subtle aspects of color pattern and body form distinguish these species. Dipsas andiana has a more variable dorsal pattern than D. nicholsi , at least in the small samples available. For example, the shape of the blotches and the width of the interspaces vary more in our series of D. andiana than in D. nicholsi (compare figs. 13–15 with fig. 5). In general, the blotches in D. nicholsi are more rounded than those in D. andiana . The relative width of the interspaces varies along the length of the body in D. andiana : posterior interspaces are broader relative to the blotches than are the anterior ones. In contrast, the blotches and interspaces are more similar along the length of the body in D. nicholsi . In part, this difference corresponds to the generally narrower posterior blotches (compared to anterior ones) in D. andiana , whereas blotches are a more uniform shape and size the entire length of the body in D. nicholsi . Adult Dipsas nicholsi have a more foreshortened and rounded head than D. andiana (head longer and narrower; compare fig. 4 with figs. 12 top and 15 bottom), and D. andiana has a rather more compressed body than D. nicholsi .

Dipsas andiana and D. nicholsi also differ in hemipenial morphology, with the caveat that we have examined only a single organ of D. andiana and cannot exclude the possibility of some variation. The enlarged spines encircling the organ are straighter and more slender in D. andiana than in D. nicholsi , and the battery comprises only a single row across on most of the organ in D. andiana , compared with 3–4 rows in D. nicholsi . The reduced calyces observed between the branches of the sulcus spermaticus in D. andiana were not observed in D. nicholsi , which has well­developed calyces in this area. Dipsas andiana has more distinct lobes on each side of the nude pocket than does D. nicholsi . Otherwise, the hemipenes of these two species are similar to one another and to other Dipsadini (references cited under D. nicholsi ).

BIOGEOGRAPHY: The above differences in scutellation, color pattern, body form, and hemipenes justify the recognition of Dipsas nicholsi and D. andiana as separate species. The two are widely allopatric, appearing to be absent in the intervening area of western Colombia and eastern Panama (map 3). The distribution of Dipsas nicholsi appears exceptionally small for a lowland forest snake, but there are several other endemic snakes at low and moderate elevations in central Panama (Myers, 2003).

Because of strong similarities in body form and color pattern, especially the peculiar head marking, it is tempting to suggest that Dipsas nicholsi and D. andiana might be sister species. The unusual head marking seems uniquely shared by the two species and is a potential synapomorphy. However, in the absence of other corroborating characters or a more explicit phylogenetic analysis of Dipsas , we are not overly confident about this interpretation. The hemipenes of D. andiana and D. nicholsi , although similar, are also in many respects comparable with those of other Dipsadini (see above discussions of hemipenial morphology); they provide no unambiguous synapomorphies exclusive to D. andiana and D. nicholsi .

We are intrigued by the conjunction of unusual characters shared by Dipsas nicholsi and D. andiana , and by their geographical distribution. Many taxa of amphibians and reptiles reputed to comprise groups of closely related species, if not sister species pairs, are distributed in patterns roughly similar to that of D. nicholsi and D. andiana —with a separating gap in eastern Panama and/or western Colombia.

A few examples include frogs of the the Dendrobates histrionicus group (Myers et al. 1984), the genus Phyllobates (Maxson and Myers, 1985) , the Eleutherodactylus fitzingeri group (Lynch and Myers, 1983), and E. bufoniformis and E. necerus (Lynch and Duellman, 1997) ; and snakes of the Coniophanes dromiciformis group (Cadle, 1989), and Dipsas viguieri and D. gracilis (Peters, 1960a: 48) . The pattern is not restricted to amphibians and reptiles, as shown by examples from birds (Brumfield and Capparella, 1996), fish (Vari, 1988), and plants (Gentry, 1982). Apart from specific clades, a proportion approaching 90% of the amphibians and reptiles shared by rain forests of eastern Panama and northwestern South America is endemic (Lynch, 1979; Duellman, 1999). However, isthmian distributions reflect a complicated history, and, unlike Dipsas nicholsi , some other snakes endemic to central Panama lack such apparent close relatives in northwestern South America (Myers, 2003).

Because of their unusual shared characteristics and a disjunct distribution common to diverse faunal elements, we suspect that D. nicholsi and D. andiana may be sister species. Corroboration must await more explicit phylogenies than currently exist.

DEFENSIVE BEHAVIORS IN THE DIPSADINI

In our experience, the Dipsadini are docile snakes when handled and never attempt to defend themselves by biting, nor have we ever seen one feign a strike with either open or closed mouth. 14 Nevertheless, many species acquire a defensive head shape and some have stylized defensive posturing. Until recently, the literature has been curiously silent on defensive behavior in the Dipsadini . Greene’s (1988) important review of antipredator mechanisms in reptiles includes an extensive taxonomic index (his appendix D) that led us only to Mole’s (1924) brief report for Sibon nebulatus . Prof. Greene (personal commun.) subsequently has flagged three new references pertaining to species of Dipsas and Sibynomorphus ( Sazima, 1992; Martins and Oliveira, 1998; Marques et al.,

14 We stress ‘‘in our experience’’, not only because we have no familiarity with many species, but also because snakes are sometimes crafty and unpredictable. For example, the rear­fanged, cat­eyed snakes of the colubrid genus Leptodeira also can normally be handled without expectation of being bitten. In San Blas, Panama, however, Myers encountered two uncharacteristically ferocious specimens, one of which succeeded in biting its captor (KU 110621, L. septentrionalis ).

2001); the four literature references are incorporated below.

Unless otherwise referenced, the following commentaries are extracted from Myers’ field catalogs. Obvious defensive behavior was often recorded, but usually there was no actual attempt to evoke a behavioral response from a captive that was headed for the preserving tray. Eventually, however, it was realized that some dipsadinine behaviors exist that are never manifest in a hand­held snake, and that interesting results are sometimes obtained if a newly caught snake is placed on a substrate and deliberately provoked by a prodding finger. Thus, many snakes were poked and harassed, but many more were not, and some potential behaviors may well have been overlooked.

Furthermore, casual notes suggest that some species may show individual variation in their responses (see Sibon nebulatus below), and some species perhaps lack defensive behavior altogether. One possibility of the latter might be Sibon annulatus , for which specialized defensive behavior was not noted in five observations in Panama; fieldnotes for a specimen (KU 112472) from the mouth of the Río Concepción, Veraguas, explicitly stated ‘‘No special defense behavior except coiling passively but then taking off rapidly once it is induced to move’’. Finally, since many or most newly caught, hand­held colubrid snakes defensively void fecal material and/or anal gland secretions, this behavior usually was not recorded and is not considered further.

CHANGE IN HEAD SHAPE

The acquiring of a more­or­less triangular head shape is the most common and widespread performance by newly caught Dipsadini (and various other snakes), even when being held in hand. It makes the rear of the head much wider than the neck, which is exceptionally slender in some species (fig. 16). So widespread is this behavior among the Dipsadini that we suppose occasional predators must be deterred by it.

The process of behavioral widening or ‘‘triangulation’’ of the head for defense involves the raising and lateral spreading of the quadratomandibular articulations—a second­ ary use of the highly kinetic feeding mechanism of serpents. Since it is an easy thing for a snake to do, it is not surprising that head triangulation is practiced by a variety of snakes, including other species living together with some of the Dipsadini (e.g., see note under Dipsas sp. below). The biomechanics has been little studied except by Young et al. (1999), who demonstrated differences in the musculoskeletal mechanisms of defensive head triangulation in two notclosely related colubrids (not discussed is the possibility that adaptations for very different foods [toads vs. bird eggs] might influence the details of triangulation).

Head triangulation such as that shown in figure 16 is now known for the following species in three of the four genera of Dipsadini (we have no experience with Tropidodipsas ); head triangulation sometimes was accompanied by a false striking position, sometimes by an attempt to flee. An asterisk (*) before the name denotes additional behaviors that are discussed in subsequent paragraphs.

* Dipsas albifrons : Head triangulation is shown in photographs in Marques et al. (2001: 33, 125).

Dipsas catesbyi: Myers photographed a specimen ( MZUSP 8730 ) showing head triangulation at Santa Cruz da Serra, in Rondônia, Brazil. Cadle observed several specimens from Pando, Bolivia, that also exhibited the behavior, and one of these also flattened most of its body.

* Dipsas indica : Head triangulation occurs as part of more complex behavior (see below).

Dipsas oreas: Cadle photographed several individuals from two localities in northwestern Peru in which head triangulation was accompanied by raising the anterior part of the body and bringing the head back into an Sshaped loop (fig. 9); occasionally an additional bend was added to the loop. The posterior body was usually ‘‘anchored’’ with a loose coil. As in a D. viguieri described below, individuals would crawl while holding this position, and striking behavior could not be induced.

* Dipsas pavonina : Head triangulation is part of a defensive display (see below).

Dipsas temporalis: This is a relatively common Panamanian species usually found active in low vegetation at night. Several were found by day in bromeliads on sides of trees about 1.2–3 m aboveground in a Darién cloud forest (Myers, 1969: fig. 11). They were tightly but variably coiled, apparently sized to fit available crevices in the bromeliads. They remained in their original passive coils when first picked up—except for spreading the rear angles of the jaws. When handled more roughly they attempted to escape while still keeping the triangulated head shape. When placed on a flat surface (fig. 16 bottom) they would respond to a light touch by ‘‘an abrupt reflexive jerk’’; the head would be raised from the ground when they then attempted to flee.

Dipsas viguieri : Head triangulation was noted in a specimen from lowland Madden Forest (fig. 16 top) and in another ( KU 110316) from the upper Río Tuira in Darién, Panama. Myers’ field notes for the last specimen stated that

This snake has a threat display: the anterior part of the body is raised and the head brought back into a short S­shaped loop and the rear of the head expand­ ed; the loop is held close and short, giving considerably more emphasis to what is otherwise a slight anterior. The snake would crawl [while holding] this position (including anterior part of body raised); it couldn’t be induced to strike.

Dipsas sp : A distinctively colored, but still­unidentified specimen (AMNH 14685), from the continental divide north of El Cope´, 600 m, Cocle´, Panama, widened its head as its ‘‘only defense’’. A specimen (AMNH 115924) of Leptodeira annulata from the same locality also spread its ‘‘rear jaw articulations noticeably’’, as did a sympatric specimen of Sibon argus .

* Sibon argus: The aforementioned specimen of S. argus (AMNH 115927) widened its head (fig. 17 top) as part of more complex behavior. See below.

* Sibon nebulatus: At least some specimens of this common species exhibit head triangulation, sometimes with body coiling and head hiding. See below.

* Sibynomorphus mikanii : Head triangulation was part of a more elaborate display of body inflation, coiling, and head hiding, as discussed below.

Sibynomorphus neuwiedi: Sazima (1992: 211 , fig. 18f) considered S. neuwiedi a probable mimic of Bothrops jararaca in southeastern Brazil. His photograph shows a specimen with triangulated head ‘‘in cocked posture’’ (but not facing the camera). The defensive displays were said to ‘‘approach those described here for Dipsas [ indica ]’’, suggesting possibly a different pattern from that described below for a specimen of Sibynomorphus mikanii .

COMPLEX DEFENSIVE BEHAVIORS: HEAD TRIANGULATION, BODY INFLATION, BODY COILING, AND HEAD HIDING

In some species, head triangulation is accompanied by more elaborate body positioning.

Dipsas albifrons: Sazima (1992: 211) stat­ ed that Brazilian D. albifrons ‘‘from the Atlantic Forest mimics even better [than D. indica ] the color pattern and defensive movements of B[othrops] jararaca (pers. obs.).’’ Marques et al. (2001) indicated by use of symbols that these defensive movements include head triangulation, erratic movements, and striking with the head (presumably with closed mouth). A photograph (Marques et al., 2001: 125) also shows a specimen in a tight, nearly flat, symmetrical coil (probably the end stage of a complex defensive display).

Dipsas indica: Sazima (1992: 211 , fig. 18d) observed that in southeastern Brazil:

When found moving slowly on the ground, an individual of D. indica may easily be mistaken for a young or slender B[othrops] jararaca . This resemblance is enhanced when the disturbed snake coils, expands its body, cocks and triangulates its head ( Fig. 18d ), and thrashes the forebody as if delivering false strikes.

Martins and Oliveira (1998: 104) noted that in Amazonia all specimens handled ‘‘thrashed the body, triangulated the head (sometimes exaggeratedly), expelled cloacal gland products, and did not bite.’’

Dipsas pavonina: In Amazonian Brazil, Martins and Oliveira (1998: 105) found that, when approached, D. pavonina ‘‘usually becomes immobile and may compress the body dorsoventrally (giving rise to a triangular cross section of the body), enlarge the head, and make subtle thrashes.’’

Sibon argus : Notes and photographs (fig. 17) were obtained for a specimen ( AMNH 115927 ) from 600 m on the continental divide north of El Cope´ , Cocle´, Panama. In addition to head triangulation, there was

A ‘ coiled spring’ defensive behavior: When bothered, it started coiling body [counterclockwise] from head (no coiling from tail­end first), forming a raised coil with head concealed in center [fig. 17 bottom]. During this time it also kept rear jaw articulations spread, but made no attempt to strike and kept mouth closed. The coil was maintained even when turned upside down.

Sibon nebulatus: Most field notes on this common species lack mention of defensive display, but the following brief observations on three specimens from the eastern end of the Isthmus of Panama, Darién, suggest that there is individual variability.

A specimen (AMNH 119391) from 640 m elevation on the Río Pucuro, at the south base of Cerro Tacarcuna, ‘‘when first caught thrashed about and soon coiled into an irregular ball but would not repeat this behavior the following morning’’. (Presence or absence of head triangulation was not noted.)

A specimen ( KU 112475) from 540–560 m on the NE slope of Cerro Sapo exhibited more complex behavior :

When picked up it coiled its body and spread the

Variable posturing similar but not quite identical to the above was observed and photographed for a third Darién specimen of S. nebulatus ( KU 112474), collected at a lowland camp on the Río Jaqué (fig. 18) :

The angle of the jaws are spread widely and the snake assumes a stance with reared­back head in an S­loop much like a Bothrops . When further disturbed it forms a close coil, that is however not a symmetrical spring­shape nor done with the jerky movements of Dipsas nicholsi . It may hide its head within the coil [fig. 18 bottom] or hang it along side the coil—nose down—with the angles of the jaws still spread [fig. 18 middle]. It would not bite.

angles of the jaws wide [triangulation]. When disturbed at other times [i.e., during the next day or two before preservation] it would spread the jaws and either attempt to hide its head under coils of body, or hold the neck in a striking S­shaped curve and sometimes whip the body into a coil.

The earliest record concerning defensive behavior in any of the Dipsadini seems to be Mole’s (1924: 249) comment on Sibon nebulatus in Trinidad, given here in full: ‘‘When molested this snake draws back its head in a most threatening manner, and even strikes, but with closed jaws. ’’ We can confirm only the drawing back of the head, 15 since we have never noticed strike­feigning in this or any other species of Dipsadini (but see uncorroborated indication for Dipsas albifrons above).

Sibynomorphus mikanii: Observations were made on a specimen (AMNH 114579) lacking data, received at the American Museum in 1973, from the Bronx Zoo. When disturbed (fig. 19 top), this snake raised and widened its head while inflating its body, especially the neck, frequently curling its tail into a flat coil (tail kept dorsal side up). The body inflation resulted in lateral spreading, especially of the neck (fig. 19 top). After continued provoking the snake deflated its body, defecated, and coiled into an irregular ball, concealing its head within or under the ball (fig. 19 middle). There was no apparent consistency in this behavior; figure 19 (bottom) shows an alternative arrangement of the asymmetrical coiling. The snake would not strike or bite.

The lateral neck inflation seemingly approaches the dorsoventral flattening or hood

15 It is the essence of snakeness for a serpent to draw back its head for different reasons, including instinctive withdrawal from movement of a strange object such as a camera. Therefore, the reason for an S­shaped posture cannot always be ascertained from a photograph (e.g., the top view in fig. 1). In life, however, there is usually no mistaking the ‘‘threatening manner’’ mentioned by Mole.

or body flattening in Sibon , but dorsoventral body compression has been noted above for Dipsas catesbyi and D. pavonina .

formation of such xenodontine snakes as Heterodon and the Xenodontini . The body flattening and head positioning of the snake shown in the top part of figure 19 is also similar to behavior described for the dipsadine Ninia sebae , which also hides it head during subsequent parts of a complex display (Greene, 1975: 481). We have not seen neck

STEREOTYPIC COILING BEHAVIOR OF DIPSAS NICHOLSI

Not one of the three specimens of Dipsas nicholsi collected alive by Myers in central Panama was noted as having displayed head triangulation, although such head widening was recorded for a specimen of Dipsas viguieri (fig. 16) found nearby and on the same night as one of the nicholsi . The aforesaid specimen of D. nicholsi (KU 110313) was kept alive for awhile in Myers’ office at the Gorgas Memorial Laboratory in Panama City, where the following observations were made.

The snake was docile when held, but displayed consistent, jerky coiling behavior when it was placed on a flat surface and provoked by touching. There were essentially three stages from the stretched­out snake to the finished coil:

Stage 1: The anterior body was thrown or jerked into a counterclockwise coil, which is shown being initiated in the upper part of figure 20.

Stage 2: If touched again, the posterior body and tail were thrown into a separate coil, as shown in the lower part of figure 20.

Stage 3: When touched a third time, the snake drew itself into the final coil—in the shape of a raised spring, widest at the bottom and with the head always exposed on the top center—as shown in figure 21 (also see fig. 1). It would remain passive in this position for some time (not clocked) before uncoiling and trying to crawl away.

After several days in captivity, stage 2 above was omitted and a different behavioral sequence became evident. The snake would coil its anterior part (stage 1 above), then draw its rear half close to the forward coil, and, while maintaining the anterior coil, suddenly straighten out the rear part in a whipping motion, as though striking. If prodded more, it would go into the stage 3 coil—but it could then be induced to partly uncoil and repeat the whipping motions.

The complete stereotypic coiling sequence of Dipsas nicholsi —and its alternative rapid whipping motions while maintaining a for­ ward coil—comprise the most elaborate dipsadinine defensive behavior seen by us. The apparent lack of head triangulation in D. nicholsi also seems notable, since it is widespread among other species.

It would be interesting to test a specimen of Dipsas andiana , conceivably a sister species, for possible behavior comparable to that of D. nicholsi . An incidental field note by John D. Lynch stated that one D. andiana (KU 132503) reacted to disturbance by ‘‘whipping into a rapid ‘S’ [and its] behavior matches its color similarity to Bothrops .’’ The ‘‘whipping’’ movements might be behavioral homologues in D. andiana and D. nicholsi , but there probably is considerable convergence in such behavior. For example, some colubrid snakes of the African genus Prosymna also

produce a ‘‘watchspring’’ defensive display when disturbed... The snake forms a tight and symmetrical flat coil with the head in the centre, then violently uncoils and recoils, making it difficult to comprehend the size of the snake. (Broadley, 1980: 546–547)

The flatter coils and the rapid recoiling are obvious differences from Dipsas nicholsi . Direction of coiling might be a difference be­ tween Prosymna and the Dipsadini . Prosymna species appear (always?) to coil in a clockwise direction (see photograph and quote in Broadley, 1980: 546, pl. 4), whereas the above specimen of Dipsas nicholsi consistently coiled counterclockwise (following the coils starting at the neck from above), as also seen in photographs of Sibon argus and S. nebulatus (figs. 17–18) and Dipsas albifrons (Marques et al., 2001: 125) .

In summary, the not uncommon serpent behavior of head triangulation is widespread but not universal among the Dipsadini , and is sometimes accompanied by drawing back the head in a (false) striking position. Inflating the body while widening and flattening, a less common serpent defensive behavior, has been seen in Sibynomorphus and in a few species of Dipsas . The occasional elements of jerky or whipping body motions are conceivably distracting to a predator and might be perceived as a snake striking out. Coiling behavior—very asymmetrical in Sibynomorphus mikanii , somewhat asymmetrical in Sibon nebulatus , but symmetrical in Sibon argus , Dipsas albifrons (Marques et al., 2001: 125) , and especially in Dipsas nicholsi — with or without head­hiding, turns slender snakes into heavier, unpreylike masses.

Owing to coloration and such behavior as head cocking in false striking position, several species of dipsadinines might be mistaken for dangerous vipers, but mimicry seems not to be a driving evolutionary force in the group as a whole. Although members of the Dipsadini cannot inflict damage and doubtless are easy picking for most predators, surely all this defensive posturing cannot always be futile. At least some inexperienced and not overly hungry predators must sometimes be deterred, and any escape that permits a later, successful breeding will have been worth the effort.

Figures

Fig. 18. Defensive behavior of Sibon nebulatus from the Río, Jaque´, 1.5 km above Río Imamado´, 50 m, Darién, Panama (KU 112474). Top: An initial stage with posteriorly widened head. Middle: Passive coil with widened head exposed. Bottom: An alternative coil with head (still widened) concealed.